Monday, August 19, 2013

xBee battery powered motion sensor, part the second.

This is a followup to an earlier post as came out in the comments you can't wake up an xBee directly with the output from the motion sensor, it's the wrong polarity. It connects to Vcc when motion is detected and floats when there is no motion. Just adding a pulldown and a transistor didn't work at all to connect it to ground and wake up the xBee. Too many timing issues.

So I decided to go with an ATTiny85 chip that can sleep most of the time and be woken up itself when the sensor goes high, and it can then handle the timing of waking up the xBee and having it sample it's pins and send the data. This actually worked lovely until I tried to add back in the motion sensor.

It turns out these motion sensors just dont work below 3v or so. They have an onboard voltage regulator for the 3.3v that they need to work so you really must power them with 5v to get them going. There is some talk on teh intertubes that you can just short across the regulator on the board. I think those instructions were written before they switched to all surface mount parts on these boards which makes it much more difficult. I did try doing that though and it did work then at 3.3v quite well.

you can see my tiny little wire loop across the voltage regulator. Unfortunately at about 3v and lower it stops behaving at all which means that 2AA batteries are not going to power it for very long. And it also stops behaving above 4.5v which means that 4 batteries are a no go. The ATTiny has a good range of voltages in this area which makes it excellent for a long term battery operated product but the xBee is very sensitive to voltages above 3.3 volts. I wouldn't want to run it on 3 batteries without a regulator. Which means that would require a regulator which would waste a lot power and significantly reduce battery life.

I've looked around quite a bit, and there are some excellent canned solutions to motion sensing out there, including some fantastic chip/sensor combinations from Zilog (who also offer an SMT version of the Z80 CPU chip which somehow I find funny :) but they use too much current and still dont operate at a low enough voltage for a good battery operated project.

So we're left with separate power supplies for the sensor and the xBee or regulators or rolling it from scratch. I might experiment with a 3AA battery system where the sensor runs with it's regulator off 3 batteries and see what the current usage is and tap just 2 of the batteries to power the xBee. They will run down out of sync but that might still work and 3 batteries in the device isn't out of the question. Or it really might be time to start studying how these sensors work and roll one from scratch. The otherwise quite good except in radio range and reliability X10 motion sensors work for years on 2AAA batteries. I've glanced at their insides and I think they do a op amp comparitor thing between the 2 outputs of the motion sensor and then have a tiny PIC to make decisions on the output.

At least I understand why the newer ZWave motion sensors are so expensive and why the battery life in them sucks so badly overall.

But all this does mean that without the motion sensor the combination of an ATTiny and an xBee radio work great. So other sensors should be a piece of cake, something like a door/window contact closure device that might also sample temperature and light levels on a regular basis should be quite possible. I might concentrate on that for the moment and stew over the motion sensor problems a little longer.

Tuesday, August 6, 2013

Tankless Water Heater Musings

One way of saving money and energy that is touted greatly is a tankless water heater. As I understand it there is still some controversy over just how much money this will save you, and they tend to be rather expensive so the payback can be long. A year ago or so I strapped a temp sensor to the inlet and outlet pipes of my traditional tank water heater so I could measure it's usage and see just how much it's running when nobody is using water to give myself some idea of the use of a tankless water heater here.  Specifically they are dallas 1-wire sensors connected to a Barix, Barionet 50 and finally reading their data in to XTension.

Here is a few recent days worth of data. Yes, I know I have the temperature turned up too high, but this is the kitchen, laundry and master bedroom heater and not the one in the kids rooms. My kids are big enough now that it's not that much of a danger to them. The second water heater that serves their rooms  is still turned down to 120 though so don't worry about us.

The blue line is the cold inlet temp and the red line is the hot output line. The first thing I see is that the cold inlet temp rapidly reaches the same temp as the outlet as soon as you stop using water. That makes me suspect there isn't a heat trap or whatever that fixture is that is supposed to reduce heat loss through the pipe on that line. I'll have to investigate that. But the other thing you see is that in these 3 days there isn't a single point where the cold water inlet temp spikes without a corresponding spike in the outlet temp. Meaning that the water heater hasn't run once not in response to demand in the last 3 days. As it turns out I have to go back to July 24th to find a graph that shows a "maintenance" cycle just to keep the tank warm.

There, at 2:30 in the morning the cold temp spikes with no hot water use measured. I have to go back several weeks again from this to find another one. If I switched to a tankless heater those would go away, and any energy wasted by them. But how much would that be? 2 or 3 waste cycles a month isn't very much to pay back the high cost of a tankless water heater.

The other factor to consider is efficiency or how well does it put the energy that it uses into the water. It was my understanding years ago that electric tank water heaters are nearly 100% efficient, meaning that almost none of the energy is lost, it all goes into the water. Gas water heaters like mine do lose quite a bit of energy up the smokestack, but so do gas tankless. This is an area for further research, but I do not believe that it's a huge difference at this point. At least not enough to make me replace a currently functional hot water heater. When it's time to replace it though I will look seriously again at these figures and it may make sense in the future.

There is an additional consideration as well in cost. Gas tankless water heaters require a lot of gas, they have a minimum pipe size in order to get enough gas. At the placement in my attic where it would replace the existing water heater I would have to have new gas piping run to support a tankless of enough capacity for my home. At considerable expense. The same holds true for electric, you have to have a high amp service run to where it is, they can't just be plugged in to a handy outlet. They use a huge amount of power. If you're on a peak metering system that changes you more if you use large amounts of power your electric tankless is very likely to put you close to or over the limit so the power you use for it may be much more expensive. Do the math on this or you may be very unpleasantly surprised.

Just thinking out loud here, everyones situation is different and it may make perfect sense for you to get yourself one of these. If you use a LOT of hot water you'll never run out, and if you use very little hot water you dont waste energy keeping the tank warm. Though if the tank is like mine the number of those phantom cycles might be very few.
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